1. Technical Field
The present invention relates to a structure design of a microstrip line, and more particularly to a signal-transmission-line structure and an electronic device using the same.
2. Related Art
Currently, along with the rapid development and broad requirements of wireless communications technologies such as a wireless network, a mobile phone, a Global Positioning System (GPS), and a digital television, designing and manufacturing of a high-speed digital circuit have become an extremely hot electronic technology in the field of communications. Generally, most high-speed digital circuits apply microstrips or striplines for signal transmission, so as to transmit high-speed digital signals. Nowadays, various circuit board structures used for transmitting high-speed digital signals have been developed in the field of electronics.
In a conventional signal-transmission-line structure, two corresponding metal layers are formed respectively in two surfaces of a dielectric layer. In order to provide preferable signal transmission, the two metal layers have to be separated by at least a specific distance (that is, the dielectric layer has a specific thickness), for example, 2 μm. However, in a 65 nm CMOS process, for example, two metal layers are at most separated by 3 μm (which is a possible distance between a lowest metal layer and a highest metal layer); and in a 28 nm CMOS process, two metal layers are at most separated by 1.5 μm. The electronic device tends to be lighter, thinner, shorter, and smaller; however, to maintain the signal quality, the size of the signal-transmission-line structure is difficult to be further reduced, and therefore, the overall volume of the electronic device is limited. On the other hand, in an advanced process, metal layers and oxide layers are increasingly thinner. That induces higher resistance and higher capacitance, which makes lower Characteristics impedance (Z0) to make the design of the transmission line much more difficult.